The present invention relates to a control system for an optical fiber laser power delivery system wherein the system employs a tip assembly for the optical fibers which provides a number of advantages for the delivery system including control of beam focus and beam divergence, mechanical protection for the optical fiber and in addition, functions as the source of a condition responsive signal utilized in an associated laser monitoring and control mechanism. While providing a response for a variety of conditions, the tip assembly of the present invention responds to a temperature parameter which is dependent upon a number of conditions including optical fiber integrity, contamination at the discharge site, or the like.
The tip assembly of the present invention employs a synthetic sapphire shielding lens which, in addition to its desirable mechanical, optical and focusing properties, has a light and temperature sensitive fluorescing property useful in generating a signal utilized in a feedback control system for the laser system. With a laser functioning at a given or known power input level, the light and temperature sensitive fluorescing property of the sapphire lens or window creates a signal indicative of certain conditions existing at the tip of the optical fiber, including laser power being transmitted and tip temperature. The improved tip assembly accordingly has application not only as an optical fiber shield but as a signal source for one control and monitor portion of the laser system. The tip assembly of the present invention may be utilized in any of a variety of systems utilizing laser power, with the tip assembly being particularly well adapted for use in a laser enhanced transluminal angioplasty catheter where it functions to mechanically protect the optical fibers and also provide a feedback signal for use in monitoring the output of the laser energy at the tip of the fiber and also the temperature of the tip of the fiber.
The tip assembly and its associated control system provides a closed-loop arrangement for monitoring and controlling the output of the laser, with the closed-loop system having condition responsive signal generating means disposed at the desired delivery or treatment site within predetermined and/or preset limits. When utilized in angioplasty catheter applications, the distal portion of the system, including the improved tip assembly of the present invention, is preferably utilized as a component which may be readily coupled into and de-coupled from the other portions of the system.
The tip assembly used in the combination of the present invention employs a lens or window fabricated from synthetic sapphire or other suitable substance, with the lens being positioned concentrically with and adjacent to the distal tip end of an enclosed or sheathed optical fiber. The synthetic sapphire lens or window device emits energy in the form of fluorescent light in response to excitation with laser power, and also when heated to an elevated temperature. Since the intensity of the signal emitted by the sapphire lens in response to exposure to incident laser power is temperature dependent, the power level of the laser energy being transmitted through the fiber onto the tip and the temperature of the tip may be determined. As indicated above, the thermal condition of the sapphire lens in the tip provides an indication of the integrity of the overall delivery system as well as an indication of any anomaly which may exist in the tip area and/or the delivery system. The improved tip assembly of the present invention further provides a means to deliver or to transmit laser power to the required location, with the optical properties of the synthetic sapphire lens being capable of determining the focus or optical pattern of the fiber output, while simultaneously providing a source for a signal useful in monitoring the condition of the delivery system for the laser power, as well as the condition adjacent the exterior of the tip as represented by the thermal response or the temperature condition.
The monitor, control, and/or condition responsive feature of the present invention is enabled by utilizing a lens responsive to laser power and temperature for determining certain of the conditions existing within as well as along the optical fiber delivery mechanism as well as to conditions existing at or immediately adjacent the point at which radiation in the form of a laser beam is being discharged from the tip. In the assembly illustrated in the drawings, the inner surface or end of the lens is fixedly disposed adjacent the optical fiber tip and the outer surface or end is immersed in or otherwise subjected to the environment into which the laser beam power output is delivered. Because of its arrangement in the system, the condition responsive lens is ideally situated and suited to function as a sensor responsive to certain conditions, including laser power, or deliver system integrity and/or conditions existing in the zone receiving the laser beam power. Each of these conditions may be determined by an evaluation of the temperature of the artificial sapphire lens body through its fluorescent behavior. In addition to fluorescence of the lens as a function of laser light intensity and lens temperature, other fluorescent signals may be generated during operation of the laser, including, for example, signals generated due to fluorescence of certain materials used in fabricating an angioplasty catheter, as well as other signals obtained from materials or matter receiving laser power, including the optical cable. The most significant flourescent signal created during operation of the system is that created by the fluorescent behavior of the lens body. The signal emitted from the lens will be fed back through a portion of the laser beam delivery system, filtered, and its amplitude determined electro-optically. In other words, the amplitude of the flourescent signal obtained from the lens will be determined electro-optically, and the value obtained is compared to the value which is expected from the input to the laser. The "window" is a range of values within performance tolerances. Deviation of this signal from a window representative of an optimum signal value or range of signal values will be indicative of one or more malfunctions including, for example, unacceptable performance of the delivery system, or the existence of contamination or accumulation of debris in the zone receiving the beam energy. In operation, therefore, a sensed deviation from a certain value or window of values will normally be utilized to trigger the shut-off or interruption of flow of energy from the laser unit. A value which falls below a certain range of output values may indicate a malfunction in either the laser unit per se or in the intermediate delivery system, and may be indicative of a broken optical fiber, or fiber tip damage. Such an indication will be utilized to trigger system shut-off or the interruption of flow of energy from the laser unit.
The lens forming a portion of the tip assembly performs a number of functions in addition to the generation of a flourescent signal. For example, the lens is utilized as a focusing element for the beam of laser radiation exiting the fiber tip. This radiation is focused along a cone which converges to a focal point. In this manner, the radiation from the laser is normally dispersed in a pattern or profile representative of an expanding or diverging cone beyond the focal point. In addition, the tip assembly acts as a means to isolate or protect the fiber tip from debris build-up or corrosion when the system is being utilized in an unclean or harsh environment. When fabricated of metal or radiopaque material, it may be utilized as a tip identifier in angioplasty procedures.
In one system employing features of the present invention, a lens system is utilized which includes a synthetic sapphire lens body which functions as a sensor emitting fluorescent radiation within a certain predetermined wavelength when excited with coherent radiation from a laser source. The amplitude of the emitted signal is sensitive to temperature, so that system performance and delivery system integrity may be measured against predetermined values for various known power input levels. The emitted fluorescent radiation from this sensor has a wavelength which is detectably different from that of the incident laser radiation. The sensor, when at an appropriate operating temperature, such as within a predetermined window of operating temperatures, and while transmitting or passing a predetermined amount of laser power therethrough, emits a fluorescent radiation signal having a magnitude, intensity or level indicative of its temperature and the laser power. When the signal is within predetermined desired upper and lower tolerance limits, as determined by a comparision with a known and acceptable signal obtained from the sensor when operating at its anticipated temperature and laser power levels, the entire system is permitted to continue to function. Since the fluorescent signal, as detected, may constitute the sum of a number of such signals from components other than the tip, such as from the catheter material or the fiber optical material, the ultimate signal obtained will be the resultant or composite of a number of components or inputs. Accordingly, when the temperature level of the synthetic sapphie tip sensor is either above or below the anticipated or desired level, a fluorescent signal (having a detectably different magnitude or level) will be created which falls outside of the window. In other words, whenever a uniform and predetermined amount of laser power is being delivered to the sapphire tip, the fluorescent response is a function of the incident laser power and the temperature of the sensor. When systems employing sensors of the type described, have been calibrated, readings indicative of departures from predetermined output power levels and predetermined tip temperature levels may be detected. While the readings obtained may not readily translate into indications of absolute temperature or the like, it has been found that these readings do indicate proper operating conditions, both on the fiber side of the tip and on the exterior. Once a system has been properly calibrated, the flourescent response of the synthetic sapphire tip may be utilized as a measure of proper overall system operation.
Synthetic sapphire is an appropriate choice of lens material. The wavelength of an output beam from an Argon ion laser will normally fall within the absorption band of a chromium-doped synthetic sapphire body. It will be appreciated that synthetic sapphires containing dopants other than chromium, as well as certain other synthetic crystalline materials other than sapphire may respond in a manner such that they may be useful in systems of the present invention. However, it has been found that synthetic sapphire, slightly doped with chromium ions within a relatively low doping level, such as in the range of approximately 20 ppm provides a fluorescing signal of a useful level when exposed to high intensity laser light, such as that received from an Argon ion laser. This signal is obtained without significant loss or degradation of the laser energy passing through the synthetic sapphire. Impurities, other than the intentionally added chromium ions, are selected or controlled at minimum levels so that exceptionally low absorption and/or minimal interferring fluorescent responses are obtained upon exposure to laser radiation within the wavelength of the laser used.
The system of the present invention is designed for use with a laser beam of moderate or long duration. In this connection, however, if output pulses of known duration are employed, an indication of tip temperature may be obtained by obtaining a measurement of the decay time of the fluorescent output.
The power level of the output or response of the synthetic sapphire is inversely proportional to its temperature. Such response is typical for chromium-doped synthetic sapphire. Additionally, as the doping level of chromium ions increases, at least up to a certain range or level, the fluorescing activity also increases. It has been found that relatively low doping levels, such as in the range of 20 ppm of chromium ions, provide synthetic sapphires with a lens body having the desired fluorescent response, with this response being obtained with a minimum of losses of laser output power being transmitted through the sapphire.
While the terms "fluoresce" or "fluorescent" are used herein, it is to be noted that these terms are being used in a comprehensive sense so as to include generally the emission of light upon stimulation from another light source, and wherein the wavelength of the emitted light of the sensor element may or may not fall within the visible range for the human eye. The emitted light is normally at a wavelength longer than that of the stimulating light energy.
While the tip assembly and associated laser catheter monitor-control apparatus of the present invention has general utility for various systems employing laser beam energy, the tip assembly and its associated monitor-control is particularly adapted for use in combination with laser devices for a variety of medical end-uses, with one such end-use being in combination with laser enhanced transluminal angioplasty catheter devices. As is known, laser devices have been found useful in treatment of a number of conditions, including treatment of some forms of arteriosclerosis. Laser enhanced angioplasty catheter devices are useful for treatment of certain types of obstructions or occlusions formed, created or otherwise present in blood vessels such as those created and encountered due to plaque build-up or the like. Exposure to laser beam energy is undertaken in order to obtain either a partial removal, reduction and/or the elimination of the obstruction by means of such exposure. In such systems an optical fiber member is typically utilized for receiving, transmitting or otherwise conducting a beam of laser energy from a generator onto a remote output lens for precise delivery or discharge of the laser energy from the lens and onto or against the plaque or other matter obstructing or occluding a blood vessel. While the extent of exposure to laser beam energy may be controlled by selecting and regulating the extent of on-time of the laser, the power level involved, and/or the energy distribution at the distal tip of the fiber, it is, nevertheless, desirable to monitor the overall operation of the laser system so that any unusual variation in output power, ambient or environmental conditions may be detected, including either excessively high or unusually low power levels being delivered to the tip. The system of the present invention provides a means for determining the operating condition of the laser beam generator, its transmission system, and its operating ambience, by precisely determining the laser power output and the magnitude of the temperature change existing at the output end, thereby providing a means for alerting the user to the existence of certain operational anomalies. By way of example, operational anomalies may occur if the generator is driven with an unusually high or unusually low amount of input power, with another operational anomaly occurring if one or more beam transmitting fibers become cracked, fractured or displaced from the surface of the lens tip.
When utilized as a laser catheter monitor-control system in combination with laser enhanced transluminal angioplasty catheter devices, the apparatus of the present invention is useful in combination with those certain laser catheter control and connecting apparatus disclosed and claimed in co-pending applications Ser. No. 679,633, filed Dec. 10, 1984, and Ser. No. 679,920, filed Dec. 10, 1984, each being assigned to the same assignee as the present invention and reference may be made to the disclosure contained therein.
The embodiment of the tip assembly and monitor-control system of the present invention as illustrated herein is shown as functioning within a laser enhanced transluminal angioplasty system, it being understood that such a system is illustrated for purposes of explaining the fundamental operational features of the system and not by way of limitation.